The present invention relates generally to oscillator circuits, and more particularly, relates to a ring oscillator with enhanced, fast startup and shutdown.
Ring oscillators are used in a variety of circuits to generate clocking signals. Ring oscillators typically include a series of devices or stages connected together to form a ring with a feedback path provided from the output of a last of the series of devices to an input of a first of the series of devices. The devices may include logic gates, inverters, differential buffers, or differential amplifiers, for example. Any odd number of inverting stages with sufficient gain and offset oscillate when connected in a ring, while an even number of inverting stages simply lock on a particular starting logic level. The ring oscillator is essentially a series of stages, each stage having an intrinsic delay from input to output. The frequency of the ring oscillator output is a function of the total delay time of the series of stages. The steady state frequency of the oscillator is dependent on the intrinsic delay of each of the stages.
FIG. 1 illustrates one known ring oscillator. This prior art ring oscillator of FIG. 1 includes a series of seven differential buffers or differential gain stages connected in a loop, as shown. Each of the buffers is referenced with a voltage source VREF and includes disable and disable bar signal inputs DSBL, DSBLB. Each of the buffers is connected in an inverted configuration.
A problem of typical ring oscillators is noise that is created when switching. It is often desirable to have a ring oscillator that can be disabled, as shown in FIG. 1. When a typical ring oscillator is started after being shut down, the ring oscillator relies on gain and offset, or perhaps noise for oscillator start up. In an ideal case, there is no offset in any of the differential gain stages. Even with gain, if there is no offset on the input of a differential gain stage, no differential voltage appears on the output of a single stage. If there is no switching voltage throughout any of the stages of the ring oscillator, then there is no oscillation.
In the non-ideal case, a small voltage output offset Vo exists in each of the differential gain stages. With a gain A, an input offset Vo is amplified by gain A, and the next stage receives differential voltage A*Vo. With n stages, a last of the n stages receives differential voltage An*Vo. This is the differential voltage that is fed back from the last stage to the first stage in the ring oscillator and is amplified until it reaches the full steady state oscillation switching voltage amplitude and frequency.
Noise also can start the ring oscillator. In general, ring oscillators start if the gain is sufficient and an odd number of inversions exists. However, a problem with typical ring oscillators is the delay in reaching full differential switching voltage and also the delay in reaching steady state oscillation frequency.
A need exists for a bipolar ring oscillator with enhanced, fast startup and shutdown.
A principal object of the present invention is to provide a ring oscillator with enhanced, fast startup and shutdown. Other important objects of the present invention are to provide such ring oscillator substantially without negative effect; and that.overcome many of the disadvantages of prior art arrangements.
In brief, a ring oscillator with enhanced, fast startup and shutdown is provided. The ring oscillator includes a series of a plurality of inverting stages connected in a loop. The plurality of inverting stages includes a first multiplexer stage. The first multiplexer stage includes a first signal input, a second signal input and a select input. An oscillator feedback signal is applied to the first signal input of the first multiplexer stage. A startup circuit is coupled to the first multiplexer stage. The startup circuit includes a signal coupled to the second signal input of the first multiplexer stage for starting the ring oscillator.
In accordance with features of the invention, in a bipolar ring oscillator the startup circuit applies a full differential switching voltage signal to the second signal input of the first multiplexer stage for starting the bipolar ring oscillator after one delay of the series of the plurality of inverting differential stages connected in the loop. The startup circuit includes a fast transition run signal and a multiplexer select signal responsive to the run signal. The multiplexer select signal is coupled to the select input of the first multiplexer stage for controlling the first multiplexer stage to select the oscillator feedback signal or the static differential signal for stopping the bipolar ring oscillator.